Method of producing conductive coating on a surface and the coated article



United States Patent O METHOD OF PRODUCING CONDUCTIVE COAT- ING N ASURFACE AND THE COATED AR- TICL David G. Young, North Woodstock, Conn.,assiguor to American Optical Company, Southbridge, Mass., a voluntaryassociation of Massachusetts Application March 5, 1951, Serial No.214,023

2 Claims. (Cl. 117-411) This invention relates to improvements inelectrical conductive coatings and relates particularly to the provisionof clear transparent, durable and stable coatings of this nature,methods of making and applying said coatings, and the production ofarticles having such coatings thereon.

One of the principal objects of the invention is to provide electricalconducting coatings adapted to be applied to articles normallysusceptible to static charges, and method of making and applying thesame whereby substantially no static charge accumulation will take placeon said articles.

Another object is to provide coatings of the above character which,while retaining desirable electrical conducting characteristics, may becontrolled as to their ultimate thickness whereby the said coatings willintroduce desirable light modifying characteristics.

Another object is to provide coatings of the above character which maybe applied either in the form of single or multiple layers whereby theelectrical conductive characteristics thereof may be varied as desiredwhile still retaining durable, stable, clear and transparentcharacteristics.

Another object is the provision of a transparent conductive coating ofthe above character which is conductively efcient after prolongedexposure to moisture and subsequent drying, and after prolonged heatingat elevated temperature with high or low relative humidity.

Another object is to provide coatings of the above character which willretain their desired characteristics at elevated temperatures and at lowrelative humidity and which are resistant to mechanical shock.

Another object is to provide coatings of the above character which maybe uniformly applied in a simple, etiicient and economical manner, whichwill be resistant to abrasion, discoloration, ageing and atmosphericinnences.

Another object is to provide coatings of the above character which maybe applied to glass, plastics or other materials either transparent oropaque in nature and which will have no detrimental effect upon saidmaterials.

Other objects and advantages of the invention will become apparent fromthe following description.

ln the drawing:

Figs. l through 5 illustrate in diagrammatic form various embodiments ofthe invention.

The prior art teaches the use of surface coatings for reducing orincreasing reflections of light, for protecting the article fromabrasion and the like, or for providing conductivity. However, it isknown that reflection reduction coatings must generally be transparentsuch as in the case of optical elements through which an observer mustView. In some cases, a static charge may accumulate on the surface ofthe articles, which static charge is dispelled very slowly, if at all.This is a detrimental factor when, for example, a sensitive instrumentis provided with a transparent cover which may pick up static.

2,761,797 Patented Sept. 4, 1956 Such static, which may be applied tothe cover such as, for example, when the cover is dusted or polished,may affect the eiciency of many instruments particularly those of thetype embodying sensitive needle-type indicating means. It is also knownthat some reection reduction coatings do inherently possess an amount ofconductivity. However, in many conditions of use, such conductivity isnegligible and the static will be dispelled too slowly to besatisfactory. Also, at relatively high temperatures, the conductivitycharacteristics generally become less effective.

Known conductive coatings of the most desirable and etiicient type wererelatively opaque or lacked durability.

The present invention is directed to the provision of coatings of theabove character which possess a high degree of transparency, durabilityand stability and which will retain their conductivity characteristicsunder a wider range of use than has hitherto been possible and which mayor may not possess reection modifying characteristics.

A further advantage of the present invention is that the coatings may beapplied in a simple, convenient and inexpensive manner.

The coatings are normally applied to transparent articles havingcharacteristics of accumulating a static charge but may be also appliedto articles of an opaque nature possessing similar characteristics. Sucharticles may be formed of glass, plastics, artificial resinousmaterials, ceramics or the like. The coatings may be in the form ofsingle coatings or multiple coatings depending upon the light modifyingcharacteristics and electrical conducting characteristics desired.

Coatings of the above character which have been found to produce theresults desired may be formed in various ways. One such is illustratedby Fig. l and comprises a single layer.

For example, a solution containing from l to l0 per cent by volume oftetraethylorthosilicate, 1 to l0 per cent by volume of concentratedhydrochloric acid, 0 to 60 per cent by volume of ethyl acetate, and thebalance of ethyl alcohol, has been found to provide reflection reductioncharacteristics and results in a stable water insoluble coating.

Such a coating possesses to a certain extent conductive properties, butthese conductive properties deteriorate with increasing time and vary tosome extent with the relative humidity of the environment.

In order to provide the coating with conductive characteristics, aningredient is inserted in the original coating solution, whichingredient is characterized by its solubility in the solvent and by itsability to remain transparent in the finished coating. It has been foundthat such ingredients are those which are particularly capable ofabsorbing and retaining moisture at elevated temperatures.

Satisfactory ingredients for this purpose are lithium chloride and otherlithium salts such as acetate, sulphate, nitrate, etc.

A preferred solution for forming a transparent conductive reectionreduction coating comprises approximately l0 parts oftetraethylorthosilicate, 42.5 parts of ethyl acetate, 42.5 parts ofdenatured or ethyl alcohol, 5 parts of concentrated hydrochloric acid,to which stock solution is added from 0.1% to 0.5% by weight of lithiumstearate. Amounts of lithium stearate greater than 0.5% may be usedwhere transparency is not essential but when used for coatings ontransparent articles, this is not desirable since the coating tends tobecome cloudy. ,l

The foregoing formula gives a clean, colorless', stable, clear,transparent solution with the lithium steaate being soluble therein. Thearticle to be coated may be immersed in the solution, removed andimmediately spun to remove excess liquid. The resultant thickness iscontrolled at the time of spinning so that if eflicient reflectionreduction characteristics, for instance, are desired the thickness willpreferably be uniform and of an odd multiple of a quarter wave length ofthe incident light. Where reflection reduction is not essential thecoatings may be thicker and may be formed as single or multiple layers.However, for best results the thickness should be sutiicient tointroduce the required conductivity. Coatings over one-half wave lengthin thickness have not generally been found to be practical. The coatedarticle is then heated slightly to accelerate the insolubilization ofthe coating, the temperature depending upon the heat resistance of thearticle, lower temperatures requiring longer heating cycles to obtainthe desired results. The article is then subsequently cooled. Thisprocedure renders the coating insoluble in acids or water anddiflicultly soluble in alkaline solutions.

lf the above coating is placed on a glass article wherein the glass hasan index of refraction of 1.52, the coating will reflect only about 2%as compared to approximately 4% reflection without the coating.

The conductivity of the coating, due to the inclusion of thc conductivematerial or hygroscopic material, is such that inclusion of only 0.1% oflithium stearate or lithium chloride, for example, will cause anelectroscopc to discharge almost instantly. Even traces of the mostefficient of the added ingredients will cause an electroscope todischarge but at a slower rate. Greater amounts of the added ingredientare even more conductively eficient for the presently describedreflection reduction coating up to 5%, depending on the particularingredient used. More than 5% will cause the coating to become cloudy,as stated hereinbefore, and, therefore, certain desired opticalcharacteristics are lost.

Slight variations in the foregoing coating process may be employed withsatisfactory results. For example, after spinning, the article may beheld at room temperature in normally moist air for about one minute andthen subjected to the action of water for any desired period of time.This results in a good reflection reduction coating and gives a somewhatmore porous surface. However, the coated article should be subsequentlyheated.

ln the original solution mentioned above, other alkyl silicates such astetramethylorthosilicate, or other esters of silicic acid may be usedinstead of tetraethylorthosilicate; also other acids such as nitric,sulphuric, hydrouosilic, uoboric, hydrouoric and hydrobromic may besubstituted for the hydrochloric acid. While acids are desirable incertain cases where acidity may be undesirable, the acids may becompletely eliminated and in such cases the coating-producing solutionshould be aged for several days. The solvents, denatured alcohol andethyl acetate may be replaced by most organic solvents in which thesilicon ester and the acid are both soluble, such as methyl acetate,methyl alcohol, and isopropyl-alcohol. Another acceptable formula isapproximately 24% of ethylene glycol mono ethyl ether, of ethyleneglycol mono butyl ether, 16% of butyl alcohol, 34.9% of denaturedalcohol, 10% of tetraethylorthosilicate, 5% of concentrated hydrochloricacid and .1% of lithium stearate or other selected ingredient forproviding increased conductivity.

lt is desirable to use conductivity increasing materials which willmaintain their effectiveness at relatively high temperatures and varyinghumidities.

It is to be understood that substances may be included in the solutionwhich may be subsequently leached out by suitable solvent treatment tomodify the porosity of the coating. Such substances, for example, areurea, glycerine, calfein hydrochloride, calcium chloride and many otherswhich are soluble in the solvents used in making up the solution andwhich remain uniformly and minutely dispersed throughout the resultantcoating and which may be subsequently removed by another solvent such aswater. It must be understood, however, that such leaching out must becontrolled so as not to render uncontinuous the conductive chain formedby the uniformly and minutely dispersed ingredient which is included forproviding the desired conductivity.

Such a coating as described above has a high intrinsic hardness and,therefore, is resistant to abrasion. This is particularly advantageouswhen used on plastic and resinous materials which are relatively soft.

A desirable reflection increasing coating such as illustratcd in Fig. 2can be formed from a solution made as follows: To 75 parts of 190 proofethyl alcohol are added slowly and with constant stirring 25 parts oftitanium tetra-chloride. The reaction is rather violent and copiouswhite fumes are evolved. There results a pale yellowish colored liquidwhich no longer fumes but which is rather acid and should be stored inglass. This is a master solution which upon suitable dilution willproduce the results desired. To obtain surfaces of high reflectivity,the solution is diluted with an equal part of the alcohol. To thissolution, in order to obtain the desired conductive properties, is addedfrom .1% to 5% of lithium stearato or other selected ingredient torender the resulting coating increasingly conductive.

Application of this solution to an article is by dipping, immer-sing,and subsequent spinning or allowing to drain, or similar methods, thesolvents being evaporated from the solution, leaving upon the surface ofthe article a uniformly thick coating of the nature desired. Followingthis, the article is baked from 50 to 200 C. or more and results in acoating which, for example, on 1.523 index glass produces reflections ofthe order of 15% where the reflectivity of the glass is normally about4%.

The thickness of this coating is controlled by varying the concentrationof the active material in the master solution, by varying the dilutionof the master solution, or by varying the rate of withdrawal of thearticle from the solution after immersion, or by varying the speed ofrotation during spinning. Surface coatings of the nature describedresulting from the above solution or modifications thereof are extremelypermanent and resistant to abrasion, weathering, and chemicals.

A coating such as the alcohol-titanium coating described immediatelyabove may, if desired, be applied over a coating such as the siliceouscoating described hereinbefore. This as illustrated in Fig. 3 providesavery efficient reflection increasing coating having good conductivity,permanence, clear transparency, and abrasion rcsistance.

In some cases, if it is desired to produce a conducting coating whoselow reflection properties are greater than those produced by using asolution of tetraethylorthosilicate alone, it is possible to deposit alow index layer in superimposition with the high index layer asillustrated in Fig. 4. This can be done by applying a high index layer,such as the titanium oxide layer, which may or may not contain thematerials for increasing conductivity, of approximately 1A wave lengththickness directly on the substrate and then applying to this high indexlayer, a siliceous layer containing materials for increasing theelectrical conductivity of a thickness of substantially 1A wave length,the two coatings functioning cooperatively to reduce the reflection oflight to a very marked degree.

The thickness of the high index undercoat is not as critical as thethickness of the low index outer coat but by suitable dilution of thesolution used in producing the outer coat and observation of the colorof the reflected light a few trials will allow the adjustment of theouter solution to produce the most effective coating which for visiblelight is that whose reflected color at normal incidence is purple. Acoating which is too thin is reddish and a coating which is too thickappears blue.

This coating should also be baked as described above.

It has been found that for articles whose index of refraction is 1.52 ormore for maximum efiiciency the index of refraction of the high indexundercoating should be lower than that of the articles. This isaccomplished by mixing the two solutions in varying proportions, thecoating resulting from such mixtures being lower in index as theproportion of the tetraethylorthosilicate in the resulting solution isincreased. For example, a mixture containing 75 parts of ethyl alcohol,3 parts of titanium tetra chloride, 18 parts of ethyl acetate, 2 partsof tetraethylorthosilicate and 2 parts of hydrochloric acid, whenapplied to glass in such a manner as to produce a coating substantially1A wave length in thickness after baking has an index of refractionlower than that produced by the solution of titanium tetrachloridealone. Upon the subsequent application on this coating of the low indexlayer resulting from a solution containing the tetraethylorthosilicateand having the thickness of 1/4 wave length substantially the samereflectivity is obtained for materials having indices of refractionwhich may vary from 1.52 to 1.70.

By applying these coatings in the reversed order (all of the coatings orthe outer coating alone having the conductivity-increasing materialsincorporated therein), surfaces having increasing reilectivity over thatproduced by the single high index layer alone may be produced.

By repeating the process of forming alternate low and high indexcoatings of the proper thickness the amount of reflectance may beincreased. Thus, it is possible, by building up coating of alternatinglow index and high index, to obtain very high reflectances, of the orderof 90% for a particular wave length. For example, using a coatingconsisting of eight layers alternating low index and high index asillustrated in Fig. 5 it has been possible to produce a glass articlehaving a reflectance of over 90% at a wavelength of 500 millimicrons.The actual location of the maximum will depend upon two factors, thephysical thickness of each coat and within limits the index ofrefraction of each coat, the controlling factor being the opticalthickness which is the thickness divided by the index of refraction ofthe resulting layer.

Any of the aforementioned types of solutions may serve as a carrier fora coloring agent if desired.

An article having a conductive coating of the character describedhereinbefore when mounted so as to have a lead or ground in engagementtherewith, Will be found to be entirely free of static charges. Anystatic which is normally accumulative on the surfaces of the article inthe absence of any conductive coating will be promptly conducted away.

The conductivity of the coating should be such that when the coating isgrounded a static charge will be dispelled almost instantly.

It is also to be understood that although the carrier solutionsmentioned hereinbefore are for forming coatings which are capable ofmodifying light reflections from the surfaces of the articles to whichthey are applied, any available coating solutions can be used whichcontain a solvent in which the added ingredient is soluble and whereinthe added ingredient may be substantially uniformly dispersed. However,in accordance with this invention the added ingredient preferably shouldpossess clear transparent characteristics which will not affect thelight transmission and reflection characteristics of the originalsolutions. The added ingredient should also be capable of conducting atrelatively high temperatures and in varying humidities as described.

lt is further to be understood that although the articles referred tohereinbefore on which the conductive coatings are applied have beenformed of dielectric materialsin general, metals or synthetic mineralssuch as mica, quartz, and in general materials of extremely lowelectrical conductivity can be provided with the presently describedcoatings. In certain cases metals having dielectric coatings thereon,for example, lacquers, enamels and anodic coatings such as aluminumoxide on aluminum articles by chemical and electrical treatments mayalso be advantageously provided with conductive coatings of thecharacter described.

In providing conductive coatings wherein light modifying characteristicsare unimportant, critical control of the thicknesses of the coatings isunnecessary, it being necessary that the thickness be merely suicient tointroduce the conductivity required and controlled so that the coatingswill tenaciously adhere to the surfaces 'on which they are applied.

From the foregoing description, it will be seen that all of the objectsand advantages of the invention have been accomplished.

While the novel features of the invention have been described and arepointed out in the annexed claims, it will be understood that variouschanges in the formulae and other details of the invention can be madewithout departing from the scope of the invention, and it is intendedtherefore that all matter contained in the foregoing description beinterpreted as illustrative and not in a limiting sense.

Iclaim:

l. A coated article of the character described comprising a substratehaving a normally static chargeable `surface and a thin transparent,durable, stable electrical r`charge conductive coating thereon, saidcoating being formed of a material selected from the group consisting ofsilicon dioxide, titanium oxide, and mixtures thereof .havingsubstantially uniformly dispersed therein from one to two-tenths of oneper cent lithium chloride, said coating being of a porous nature such asto more readily permit moisture vapor to gain access to said dispersedlithium chloride so that, when properly grounded, the coating will beincreasingly conductive to electrical charges, said coating furtherbeing resistant to abrasion, shock, discoloration and aging undervarying heat and atmospheric influence.

2. The method of producing a conductive coating on the surface of -anarticle comprising the steps of apply- ,ing to said surface a liquidsolution embodying from about 1 to 10 per cent by Volume oftetraethylortho- 1silicate, about .11 to 10 per cent by volume ofconcentrated hydrochloric acid, about one to two-tenths of one per centlithium chloride, and the balance a readily volatile, water miscibleorganic solvent, causing the liquid solution to form on said surface ofthe article a liquid layer of a substantially uniform thickness, andevaporating the solvent from said liquid layer whereby the resultantlayer will be of a porous nature and will have uniformly dispersedtherein lithium chloride, the proportion of said lithium chloride beingso controlled that its presence in the coating formed will substantiallyunatfect the light transmission characteristics thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,570,077 Pitman Ian. 19, 1926 2,327,460 Rugeley Aug. 24, 1943 2,346,483Goss Apr. 11, 1944 2,366,516 Geffecken et al. Jan. 2, 1945 2,412,496Dimmick Dec. 10, 1946 2,428,357 Cohen et al. Oct. 7, 1947 r 2,466,119Moulton et al. Apr. 5, 1949 6') 2,478,385 Geist-.r Aug. 9, 19492,519,722 Turner Aug. 22, 1950 2,522,531 Mochel Sept. 19, 1950 2,601,124Moulton June 17, 1952 FOREIGN PATENTS 473,943 Great Brita-in Oct. 22,1937

1. A COATED ARTICLE OF THE CHARACTER DESCRIBED COMPRISING A SUBSTRATEHAVING A NORMALLY STATIS CHARGEABLE SURFACE AND A THIN TRANSPARENT,DURABLE, STABLE ELECTRICAL CHARGE CONDUCTIVE COATING THEREON, SAIDCOATING BEING FORMED OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OFSILICON DIOXIDE, TITANIUM OXIDE, AND MIXTURES THEREOF HAVINGSUBSTANTIALLY UNIFORMLY DISPERSED THEREIN FROM ONE TO TWO-TENTHS OF ONEPER CENT LITHIUM CHLORIDE, SAID COATING BEING OF A POROUS NATURE SUCH ASTO MORE READILY PERMIT MOISTURE VAPOR TO GAIN ACCESS TO SAID DISPERSEDLITHIUM CHLORIDE SO THAT, WHEN PROPERLY GROUNDED, THE COATING WILL BEINCREASINGLY CONDUCTIVE TO ELECTRICAL CHARGES, SAID COATING FURTHERBEING RESISTANT TO ABRASION, SHOCK, DISCOLORATING AND AGING UNDERVARYING HEAT AND ATMOSPHERIC INFLUENCE.